FR3149052A1 - Turbomolecular Vacuum Pump and Insulation Kit - Google Patents

Abstract

Turbomolecular vacuum pump (1) comprising a thermal insulation device (30) surrounding a peripheral casing (19) of the stator (2) and comprising at least one intermediate insulating layer (31, 32, 33, 34) wound around the peripheral casing (19) of the stator (2) and an external insulating layer (36) surrounding the at least one intermediate insulating layer (31, 32, 33, 34) and the peripheral casing (19) of the stator (2), the at least one intermediate insulating layer (31, 32, 33, 34) being interposed between the peripheral casing (19) of the stator (2) and the external insulating layer (36), the intermediate insulating layer (31, 32, 33, 34) and the external insulating layer (36) being respectively held wound by at least one respective retaining element (35) joining their respective end edges. Figure 1

Description

Turbomolecular Vacuum Pump and Insulation Kit Technical field of the invention

The present invention relates to a turbomolecular vacuum pump. The present invention also relates to an insulation kit for a turbomolecular vacuum pump.

Technical background

Generating a high vacuum in an enclosure requires the use of turbomolecular vacuum pumps consisting of a stator in which a rotor is driven in rapid rotation, for example rotation at more than thirty thousand revolutions per minute.

In vacuum applications, particularly in the semiconductor industry, photovoltaic panels or LEDs or in thin film deposition processes, vacuum pumps convey various types of gases and evaporated substances which can, due to changes in pressure or temperature conditions or changes in the nature of chemical reactions, deposit on the internal surfaces of the vacuum pump. These deposits can cause a clearance restriction between the stator and the rotor which can cause the rotor to seize or even crash the vacuum pump.

It is known to heat the vacuum pump to prevent condensation of reaction products and thus limit the formation of deposits to increase the life of the vacuum pump.

Heated vacuum pumps typically have a sewn-in insulating belt strapped around the stator to thermally insulate the heated stator body to reduce heat loss and protect people.

However, the production of this insulating belt is relatively expensive and does not allow easy adaptation according to the configuration of the vacuum pump, in particular to adapt the shape of the belt to the position of the purge gas or cooling water circulation valves or pipes, or to the position of the electrical cable passages or the discharge port of the vacuum pump, which may differ according to the pumping applications.

An aim of the present invention is therefore to propose a turbomolecular vacuum pump at least partially resolving the drawbacks of the state of the art.

For this purpose, the invention relates to a turbomolecular vacuum pump comprising:
- a stator,
- a rotor configured to rotate in the stator,
- a heating device configured to heat the stator,
- a thermal insulation device surrounding a peripheral envelope of the stator.
The thermal insulation device includes:
- at least one intermediate insulating layer wound around the peripheral envelope of the stator, and
- an outer insulating layer surrounding the at least one intermediate insulating layer and the peripheral envelope of the stator, the at least one intermediate insulating layer being interposed between the peripheral envelope of the stator and the outer insulating layer, the intermediate insulating layer and the outer insulating layer being respectively held wound by at least one respective retaining element joining their respective end edges.

The thermal insulation device is thus easy to assemble and adapt to the various possible options of the vacuum pump by simply cutting into flat insulating panels. The general cylindrical shape of the peripheral casing of the vacuum pump stator is taken advantage of to wrap one or more pre-cut flexible insulating layers around the heated stator body. This solution is inexpensive, lower than the cost of the sewn insulating belt of the prior art. In addition, the thermal insulation device is modular, because it allows the level of thermal insulation to be easily adapted to a specific need. In particular, it is possible to increase the number of superimposed intermediate insulating layers to reduce heat losses, in particular if the space around the vacuum pump allows it. It is easy and inexpensive to adapt the thermal insulation device by moving/creating/removing cutouts in the various intermediate insulating layers depending on the options of the vacuum pump. Also, access around the vacuum pump is simplified for maintenance operators because the thermal insulation device can easily be compressed by hand. In addition, this thermal insulation device forms a mechanical protection device for the vacuum pump, useful in particular for its transport.

The vacuum pump may further include one or more of the features described below, taken alone or in combination.

According to an exemplary embodiment, the at least one intermediate insulating layer and/or the external insulating layer comprises a flexible foam and an aluminum sheet covering one side of the flexible foam.

The thermal insulation device comprises, for example, at least a first and a second superimposed intermediate insulating layer.

According to an exemplary embodiment, the end edges of the adjacent superimposed intermediate insulating layers meet along a respective closing line whose angular positions are distinct.

The end edges of at least one intermediate insulating layer may have complementary shapes forming at least one orifice in a closing line.

According to an exemplary embodiment, the stator comprises:
- a turbomolecular stator part receiving at least one stage of stator blades and in which a stage of rotor blades is capable of rotating, and
- a sleeve fixed to the turbomolecular stator part and in which a cylindrical skirt of the rotor is capable of rotating.

The thermal insulation device may comprise at least one intermediate insulating layer surrounding at least a portion of the turbomolecular stator portion and at least one intermediate insulating layer surrounding at least a portion of the sleeve.

The heating device is for example configured to heat the socket, for example to a temperature between 120°C and 200°C, such as 150°C, for example by means of one or more heating cartridges or electrical resistors received in the body of the socket.

The outer insulating layer surrounds the at least one intermediate insulating layer and the peripheral envelope of the stator, for example over the height of the sleeve and the turbomolecular stator part.

The thermal insulation device comprises, for example, a number of intermediate insulating layers arranged around the sleeve greater than the number of intermediate insulating layers arranged around the turbomolecular stator part.

The thickness of the at least one interlayer insulating layer may be between 5mm and 25mm, such as 15mm.

The invention also relates to an insulation kit for a turbomolecular vacuum pump as described above, the vacuum pump comprising a stator, a rotor configured to rotate in the stator, a heating device configured to heat the stator. The insulation kit comprises a thermal insulation device intended to surround a peripheral envelope of the stator, the thermal insulation device comprising:
- at least one intermediate insulating layer intended to be wound around the peripheral envelope of the stator, and
- an external insulating layer intended to surround the at least one intermediate insulating layer and the peripheral envelope of the stator, the at least one intermediate insulating layer being interposed between the peripheral envelope of the stator and the external insulating layer, the intermediate insulating layer and the external insulating layer being respectively intended to be kept wound by at least one respective retaining element joining their respective end edges.

The insulation kit can thus be sold as an accessory and/or delivered independently of the turbomolecular vacuum pump. For example, it is designed to be arranged around an already installed vacuum pump. Furthermore, it is thus possible for the same vacuum pump reference to have different insulation kits, each having, for example, a different number of intermediate insulating layers or intermediate insulating layers having different thicknesses, in order to best meet the space available around the vacuum pump and/or to reduce the energy consumption of the vacuum pump heating as much as possible.

Brief description of the figures

Other advantages and characteristics will appear on reading the description of the invention, as well as the attached drawings in which:

There shows a schematic axial sectional view of an example turbomolecular vacuum pump.

There shows a perspective view of an external insulating layer of the thermal insulation device of the turbomolecular vacuum pump of the .

There shows another perspective view of the outer insulating layer of the .

There shows a perspective view of the superimposed intercalary insulating layers of the thermal insulation device of the turbomolecular vacuum pump of the .

There shows the intermediate insulating layers of the flat views.

In these figures, identical elements have the same reference numbers.

Detailed description

The following embodiments are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference is to the same embodiment, or that the features apply only to a single embodiment. Single features of different embodiments may also be combined or interchanged to provide other embodiments, without departing from the scope of the invention, as defined by the claims.

"Upstream" means an element that is placed before another in relation to the direction of flow of the gas to be pumped. Conversely, "downstream" means an element placed after another in relation to the direction of flow of the gas to be pumped.

The axial direction of vacuum pump 1 is defined as the direction parallel to the axis of rotation I-I of vacuum pump 1.

There illustrates an example of the realization of a turbomolecular vacuum pump 1.

The turbomolecular vacuum pump 1 comprises a stator 2 in which a rotor 3 is configured to rotate at high speed in axial rotation, for example rotation at more than thirty thousand revolutions per minute, so as to drive gases to be pumped in a gas flow path interposed between the stator 2 and the rotor 3.

In the example of realization of the , the turbomolecular vacuum pump 1 is called hybrid: it comprises a turbomolecular stage 4 and a molecular stage 5 located downstream of the turbomolecular stage 4 in the direction of circulation of the pumped gases (represented by the arrows F1 on the ). The pumped gases enter through the suction port 6, first pass through the turbomolecular stage 4, then the molecular stage 5, to then be evacuated towards a discharge port 7 of the turbomolecular vacuum pump 1. In operation, the discharge port 7 is connected to a primary pumping.

In the turbomolecular stage 4, the rotor 3 comprises at least two stages of blades 9 and the stator 2 comprises at least one stage of fins 10. The stages of blades 9 and fins 10 follow one another axially along the axis of rotation II of the rotor 3 in the turbomolecular stage 4. The rotor 3 comprises, for example, more than four stages of blades 9, such as between four and twelve stages of blades 9 (seven in the example illustrated in the figure). ).

Each stage of blades 9 of the rotor 3 comprises inclined blades which extend in a substantially radial direction from a hub 11 of the rotor 3 fixed to a drive shaft 12 of the vacuum pump 1, for example by screwing. The blades are regularly distributed around the periphery of the hub 11.

Each stage of fins 10 of the stator 2 comprises a crown from which extend, in a substantially radial direction, inclined fins, distributed regularly around the inner periphery of the crown. The fins of a stage of fins 10 of the stator 2 engage between the blades of two successive stages of blades 9 of the rotor 3. The blades 9 of the rotor 3 and the fins 10 of the stator 2 are inclined to guide the pumped gas molecules towards the molecular stage 5.

The stator 2 comprises a turbomolecular stator part 20 receiving the at least two stages of fins 10. This turbomolecular stator part 20 is open at one end on the suction port 6 of the vacuum pump 1. It can comprise an annular inlet flange 8 surrounding the suction port 6 to connect the vacuum pump 1 to an enclosure whose pressure is to be lowered.

Here, in the molecular stage 5, the rotor 3 further comprises a cylindrical skirt 13, called the Holweck skirt, downstream of the at least two blade stages 9, formed by a smooth cylinder, which rotates opposite a sleeve 22 of the stator 2, called the Holweck stator, fixed to the turbomolecular stator part 20 and in which helical grooves 14 are formed. The helical grooves 14 are arranged one above the other. For example, there are between three and ten helical grooves 14, such as six. The helical grooves 14 of the stator 2 make it possible to compress and guide the pumped gases towards a discharge of the vacuum pump 1 formed in the sleeve 22 and opening through the discharge orifice 7.

The turbomolecular stator part 20 and the sleeve 22 follow one another axially along the axis of rotation I-I of the rotor 3. The external surface of the turbomolecular stator part 20 and of the sleeve 22 forms the peripheral casing 19 of the vacuum pump 1 and has a generally cylindrical shape.

The rotor 3 further comprises an internal bowl 15, coaxial with the axis of rotation I-I, formed under the skirt 13 and arranged opposite a bell 17 of the stator 2, a base of which is fixed to the sleeve 22, the bell 17 projecting under the rotor 3. In operation, the rotor 3 rotates in the stator 2 without contact between the internal bowl 15 and the bell 17.

The rotor 3 may be made of a single piece (monobloc) or it may be an assembly of several parts. For example, it is made of aluminum (or aluminum alloy) and/or nickel. It may have a coating, such as nickel, in particular to better resist corrosion. For example, it is made of nickel-plated aluminum.

The rotor 3 is driven in rotation in the stator 2 by a motor 16 of the vacuum pump 1. The motor 16 is for example arranged in the bell 17 of the stator 2, itself arranged under the internal bowl 15 of the rotor 3, the drive shaft 12 passing through the bell 17 of the stator 2.

The rotor 3 is guided laterally and axially by magnetic or mechanical bearings 18 supporting the drive shaft 12 of the rotor 3, located in the stator 2. There are for example first bearings 18 supporting and guiding a first end of the drive shaft 12 in the base of the bell 17 of the stator 2 and second bearings 18 supporting and guiding a second end of the drive shaft 12 arranged at the top of the bell 17.

Other electrical or electronic components can be received in the bell 17 of the stator 2, such as position sensors.

The vacuum pump 1 comprises a heating device 21 configured to heat the body of the stator 2, in particular the sleeve 22, for example to a temperature between 120°C and 200°C, such as 150°C, for example by means of one or more heating cartridges or electrical resistors received in the body of the sleeve 22.

The vacuum pump 1 further comprises a thermal insulation device 30 surrounding the peripheral casing 19 of the stator 2.

The thermal insulation device 30 comprises at least one intermediate insulating layer 31, 32, 33, 34 wound around the peripheral casing 19 of the stator 2 and an external insulating layer 36 surrounding the at least one intermediate insulating layer 31, 32, 33, 34 and the peripheral casing 19 of the stator 2, the at least one intermediate insulating layer 31, 32, 33, 34 being interposed between the peripheral casing 19 of the stator 2 and the external insulating layer 36.

The outer insulating layer 36 surrounds the at least one intermediate insulating layer 31, 32, 33, 34 and the peripheral casing 19 of the stator 2 here over the height (in the axial direction) of the sleeve 22 and the turbomolecular stator part 20. The outer insulating layer 36 thus covers the heated part of the vacuum pump 1 from top to bottom.

The at least one intermediate insulating layer 31, 32, 33, 34 and/or the external insulating layer 36 comprise, for example, respectively a flexible foam and a waterproof and/or reflective sheet, such as an aluminum sheet, covering one face of the flexible foam. The foam is, for example, made of cellular synthetic material, such as a polymer, such as polyether. The foam forms a good thermal insulator and the aluminum sheet ensures the sealing of the foam and forms a thermal screen.

The thickness of the at least one intermediate insulating layer 31, 32, 33, 34 is for example between 5 mm and 25 mm, such as 15 mm. The thickness of the different superimposed intermediate insulating layers 32, 33, 34 may be identical or different, in particular to adapt the thermal insulation device 30 to the available volume.

The thickness of the at least one external insulating layer 36 may be less than the thickness of the intermediate insulating layer 31, 32, 33, 34, such as at least twice less.

The outer insulating layer 36 can also be formed from a flexible foam and an aluminum sheet.

The thermal insulation device 30 may comprise a semi-rigid or rigid shell surrounding the external insulating layer 36, such as a plastic part, for example curved on the external insulating layer 36.

Each intermediate insulating layer 31, 32, 33, 34 and the external insulating layer 36 are respectively held rolled up by at least one respective retaining element 35, and preferably several, here four, joining the respective end edges together along a closing line 31a, 32a, 33a, 34a, 36a (visible for the external insulating layer 36 on the ).

The retaining elements 35 are for example adhesive strips or self-gripping tapes fixed to the aluminum sheets. The different insulating layers 31, 32, 33, 34, 36 can therefore be held together independently of each other.

The intermediate insulating layer 31, 32, 33, 34 and the external insulating layer are for example wound on a respective turn around the peripheral casing 19 of the stator 2 ( ).

Alternatively, at least one intermediate insulating layer 31, 32, 33, 34 goes around the peripheral casing 19 at least twice before the end edges of the intermediate insulating layer 31, 32, 33, 34 are held together by at least one retaining element 35, such that the second turn of the intermediate insulating layer 31, 32, 33, 34 overlaps the first turn of the intermediate insulating layer 31, 32, 33, 34. The vacuum pump 1 comprises, for example, a single intermediate insulating layer going around the peripheral casing 19 several times, such as between two and four times, before being fixed by the retaining elements 35.

The end edges of the intermediate insulating layers 31, 32, 33, 34 and of the external insulating layer 36 meet along a respective closing line 31a, 32a, 33a, 34a, 36a except at the location of respective orifices possibly formed by these edges.

The end edges have complementary shapes forming holes and/or straight, curved, crenellated, zig-zag or other closure lines 31a, 32a, 33a, 34a, 36a, formed for example by the end-to-end juxtaposition of the end edges. According to another example, the end edges may have complementary shapes forming closure lines by interlocking, for example via a mortise and tenon, dovetail, tongue or other assembly.

The angular positions of the closing lines 32a, 33a, 34a of the superimposed intermediate insulating layers 32, 33, 34 may be distinct. In other words, it may be provided that the closing lines 32a, 33a, 34a of two radially adjacent intermediate insulating layers 32, 33, 34 do not overlap. The non-overlapping of the closing lines 32a, 33a, 34a makes it possible to avoid thermal weak points.

The superimposed intermediate insulating layers 32, 33, 34 may be in contact with each other, with the peripheral envelope 19 and with the external insulating layer 36.

The number of intermediate insulating layers 31, 32, 33, 34 is not limiting.

The thermal insulation device 30 comprises, for example, at least a first and a second superimposed intermediate insulating layer 32, 33.

The end edges of the first insulating layer 32 may meet along a closing line 32a whose angular positioning is distinct (different) from the closing line 33a joining the end edges of the second intermediate insulating layer 33. The closing lines 32a, 33a of the first and second intermediate insulating layers 32, 33 are for example diametrically opposed ( ).

The thermal insulation device 30 may comprise at least three superimposed intermediate insulating layers 32, 33, 34, a first intermediate insulating layer 32 wound around the peripheral casing 19, here around the sleeve 22, a second intermediate insulating layer 33 wound around and on the first intermediate insulating layer 32 and a third intermediate insulating layer 34 wound around and on the second intermediate insulating layer 32 (figures 1, 3 and 4).

Since the interlayer insulating layers 32, 33, 34 are superimposed, the flat length of the second interlayer insulating layer 33 is greater than that of the first interlayer insulating layer 32, the flat length of the third interlayer insulating layer 34 is greater than that of the second interlayer insulating layer 33, and so on ( ).

The end edges of the first and third intermediate insulating layers 32, 34 may meet along a respective closing line 32a, 34a whose angular positioning is distinct (different) from the closing line 33a joining the end edges of the second intermediate insulating layer 33. The closing lines 32a, 34a of the first and third intermediate insulating layers 32, 34 are for example diametrically opposite the closing line 33a of the second intermediate insulating layer 33 ( ).

The end edges of at least one intermediate insulating layer 32, 34 may have complementary shapes forming at least one orifice 37, for example circular, in a closing line 32a, 34a ( ). A corresponding orifice 37 is provided in the second intermediate insulating layer 33, for example in its middle so as not to overlap the closing lines 32a, 33a, 34a of the adjacent intermediate insulating layers 32, 33, 34. A corresponding orifice 40 may also be provided in the external insulating layer 36 (FIGS. 2A, 2B). This orifice 37 may allow the passage of the discharge orifice 7 formed in the sleeve 2. Other clearances 38, 39 may thus be made by simple cutting in the various superimposed intermediate insulating layers 32, 33, 34 and in the external insulating layer 36 to allow, for example, the passage of valves, pipes or cables (FIGS. 3 and 4).

The intermediate insulating layers 31, 32, 33, 34 may further cover areas separated in height and the number of intermediate insulating layers 31, 32, 33, 34 may vary along the peripheral casing 19 of the stator 2.

The thermal insulation device 30 comprises, for example, at least one intermediate insulating layer 31 surrounding a portion of the turbomolecular stator portion 20 and at least one intermediate insulating layer 32, 33, 34 surrounding a portion of the sleeve 22.

The number of intermediate insulating layers 32, 33, 34, arranged around the sleeve 22, here three, may be greater than the number of intermediate insulating layers 31 arranged around the turbomolecular stator part 20, here only one. A smaller space may be left at the level of the fixing between the turbomolecular stator part 20 and the sleeve 22, this area possibly being devoid of an intermediate insulating layer between the peripheral casing 19 and the external insulating layer 36.

The intermediate insulating layers 31, 32, 33, 34 can be axially blocked by at least one axial stop 22a, 20a, for example annular, for example fixed or formed in the stator 2, for example in the sleeve 22 and/or the turbomolecular stator part 20 ( ).

Alternatively, the intermediate insulating layers 31, 32, 33, 34 can be fixed to the stator 2 by fixing elements made of a non-thermally conductive material, such as screws or plastic rods.

The intermediate insulating layer 31, 32, 33, 34 and/or the external insulating layer 36 are thus easy to assemble and adapt to the various possible options of the vacuum pump 1 by simply cutting them into flat insulating panels. The general cylindrical shape of the peripheral casing 19 of the stator 2 of the vacuum pump 1 is used to wind one or more of these pre-cut flexible insulating layers around the body of the heated stator 2. This solution is inexpensive, less than the cost of the sewn insulating belt of the prior art. In addition, the thermal insulation device 30 is modular, since it makes it possible to easily adapt the level of thermal insulation to a specific need. It is in particular possible to increase the number of superimposed intermediate insulating layers to reduce heat losses, in particular if the space around the vacuum pump 1 allows it. It is easy and inexpensive to adapt the thermal insulation device by moving/creating/removing cutouts in the different intermediate insulating layers depending on the options of the vacuum pump 1. Also, access around the vacuum pump 1 is simplified for maintenance operators because the thermal insulation device 30 can easily be compressed by hand. Furthermore, this thermal insulation device 30 forms a mechanical protection device for the vacuum pump 1, useful in particular for its transport.

Claims (11)

Turbomolecular vacuum pump (1) comprising:
- a stator (2),
- a rotor (3) configured to rotate in the stator (2),
- a heating device (21) configured to heat the stator (2),
- a thermal insulation device (30) surrounding a peripheral casing (19) of the stator (2),
characterized in that the thermal insulation device (30) comprises:
- at least one intermediate insulating layer (31, 32, 33, 34) wound around the peripheral casing (19) of the stator (2), and
- an external insulating layer (36) surrounding the at least one intermediate insulating layer (31, 32, 33, 34) and the peripheral casing (19) of the stator (2), the at least one intermediate insulating layer (31, 32, 33, 34) being interposed between the peripheral casing (19) of the stator (2) and the external insulating layer (36), the intermediate insulating layer (31, 32, 33, 34) and the external insulating layer (36) being respectively held wound by at least one respective retaining element (35) joining their respective end edges. Vacuum pump (1) according to the preceding claim, characterized in that the at least one intermediate insulating layer (31, 32, 33, 34) and/or the external insulating layer (36) comprises a flexible foam and an aluminum sheet covering one face of the flexible foam. Vacuum pump (1) according to one of the preceding claims, characterized in that the thermal insulation device (30) comprises at least a first and a second superimposed intermediate insulating layer (32, 33, 34). Vacuum pump (1) according to the preceding claim, characterized in that the end edges of the adjacent superimposed intermediate insulating layers (32, 33, 34) meet along a respective closing line (32a, 33a, 34a) whose angular positions are distinct. Vacuum pump (1) according to one of claims 3 or 4, characterized in that the end edges of at least one intermediate insulating layer (32, 34) have complementary shapes forming at least one orifice (37) in a closing line (32a, 34a). Vacuum pump (1) according to one of the preceding claims, characterized in that the stator (2) comprises:
- a turbomolecular stator part (20) receiving at least one stage of fins (10) of the stator (2) and in which a stage of blades (9) of the rotor (3) is capable of rotating, and
- a sleeve (22) fixed to the turbomolecular stator part (20) and in which a cylindrical skirt (13) of the rotor (3) is capable of rotating. Vacuum pump (1) according to the preceding claim, characterized in that the thermal insulation device (30) comprises at least one intermediate insulating layer (31) surrounding at least part of the turbomolecular stator part (20) and at least one intermediate insulating layer (32, 33, 34) surrounding at least part of the sleeve (22). Vacuum pump (1) according to one of claims 6 or 7, characterized in that the external insulating layer (36) surrounds the at least one intermediate insulating layer (31, 32, 33, 34) and the peripheral casing (19) of the stator (2) over the height of the sleeve (22) and the turbomolecular stator part (20). Vacuum pump (1) according to one of claims 7 or 8, characterized in that the thermal insulation device (30) comprises a number of intermediate insulating layers (32, 33, 34) arranged around the sleeve (22) greater than the number of intermediate insulating layers (31) arranged around the turbomolecular stator part (20). Vacuum pump (1) according to one of the preceding claims, characterized in that the thickness of the at least one intermediate insulating layer (31, 32, 33, 34) is between 5mm and 25mm, such as 15mm. Insulation kit for a turbomolecular vacuum pump (1) comprising a stator (2), a rotor (3) configured to rotate in the stator (2), a heating device (21) configured to heat the stator (2), characterized in that the insulation kit comprises a thermal insulation device (30) intended to surround a peripheral envelope (19) of the stator (2), the thermal insulation device (30) comprising:
- at least one intermediate insulating layer (31, 32, 33, 34) intended to be wound around the peripheral casing (19) of the stator (2), and
- an external insulating layer (36) intended to surround the at least one intermediate insulating layer (31, 32, 33, 34) and the peripheral casing (19) of the stator (2), the at least one intermediate insulating layer (31, 32, 33, 34) being interposed between the peripheral casing (19) of the stator (2) and the external insulating layer (36), the intermediate insulating layer (31, 32, 33, 34) and the external insulating layer (36) being respectively intended to be kept wound by at least one respective retaining element (35) joining their respective end edges.